Welcome back everyone. So, in our last little discussion we talked about just the basic idea of orders of magnitude. And we've used five orders of magnitude in dollars to understand how big a jump there was from a snickers bar, which was one dollar, to a, a possibly a nice apartment in Paris for hundreds of thousands of dollars. And that was just five orders of magnitude. Let's now apply that idea to astronomy, to understand how we can move through different size scales in the universe. So let's start with our size scale and we're going to be measuring things in meters. Now again, remember that when we talk about orders of magnitude, what we're talking about is you know, the general size of, of quantities. So for example, I'm about two meters tall. I'm a little bit shorter than two meters tall. Like to be two meters tall, didn't quite work out that way. But, I'm about two meters tall. However, in terms of order of magnitude, I am ten to the zero meters, right. That's about one meter. I'm, you know, in terms of order of magnitude, I'm about one meter tall, okay. So that's the size scale of a human, and we'll use that as our reference frame for moving forward. Now, the next large-scale, or, you know, important structure in the universe, after us, and perhaps we shouldn't even use ourselves. But you know were going to, we have to have a reference so we'll use ourselves to begin with. And the next step up in structure in the universe are planets. And planets as we have seen range in you know from our discovery range in size scale from 10 to the 6 to 10 to the 7 meters. So the difference in size scale between me and a planet is larger, the jump in orders of magnitude is larger than the jump from a snickers bar to you know the really nice hotel or the nice apartment in Paris. So that's just on the first leap in terms of size scale. The next structure that we have to deal with in astronomy are stars, and stars have size scales between say 10 to the eight and 10 to the 12 meters in scale. So there are you know, dwarf stars and there are giant stars. So, after stars, the next structure that we're interested in are planetary systems, the collection of planets that are orbiting around stars. So like our solar system, but we've also found other planetary systems, and the typical size scales there are on the order of 10 to the 13 meters, so that's a factor of 10 to a factor of 100 or even 1,000 larger than the stars themselves. The next size scale we're interested in are galaxies, and galaxies are vast collections of stars that exist in, they're gravitationally bound, and the typical size scale for a galaxy is on the order of, say, 10 to the 19 meters. Now, notice what a jump there was between the previous size scale, which was a solar system, and a galaxy. And so what we're really seeing there is approximately nine orders of magnitude from the coherent size of a solar system to the collection of solar systems or stars that go into a galaxy. So there can, there's this enormous range of, of, of size scales that we're dealing with in astronomy, and we haven't even gotten to the top in some sense. The next size scale that we're interested in are galaxy clusters. These are groups of galaxies. And typical size scales there are a 1,000 times or more the size of the galaxies. So 10 to the 22 meters. And then finally we have the clusters, this, the super clusters of galaxies which are clusters of clusters of galaxies. And there the size scales are 10 to the 24 meters, so even a thousand times, hundred times, or a thousand times more. So, then we go all the way up to the size of the universe itself. Now we can talk about the observable universe, which is the, the sort of, bubble of light that encompasses the universe that we can see. But the thing that's important to understand is there is most likely a lot more universe beyond even that. So in some sense, well, according to our best understanding, is that the universe right now, our understanding tells us that the universe is, in fact, infinite, on the largest scales. Okay, so now we've done the large scales. Now let's turn things around and go to the small scales. So, we now want to travel downward. Again, I'm a human being. You are a human being. We're both approximately one meter in size, and on the order of magnitude, 10 to the 0th meters. So, what about if we go downward? Well, think about a grain of dust. A grain of dust is something we all have some experience with because we sweep. Lots of them up every time we sweep our floors, and typical size scale for a grain of dust is about a millionth of a meter, 10 to the minus six. So there are six orders of magnitude that you have to go through from your basic size to the size of a dust grain. And those dust grains are made up of atoms, and what are the size of an atom? Well, typical size of an atom is going to be about 10 to the minus 9th of a meter, a billionth of a meter. Okay. So, now we see nine orders of magnitude between your size and the size of your basic constituents. Your atomic constituents. So, of course, we know that atoms are themselves built of even smaller particles. Things like neutrons and protons that make up the nucleus. And there's actually a whole zoo of sub atomic particles and the typical sub atomic particle. Size scale in terms of how we as when we throw things at it, when things tend to bounce back. Typical size scales are on the order of 10 to the minus 15 of a meter. So that is a full six orders of magnitude from the atomic size scale. So, another millionth or a million times smaller. So what we see is there's about 39 orders of magnitude from the smallest known structures in the universe to the largest known structures in the universe. And how far down can we go? Well, that's really an interesting question. We know that at some point when you go far enough down even space-time itself should break up into some kind of foamy structure, and we'll talk about that later on towards the end of the class. but as you can see, there is this amazing, psychotic range of size scales, both on the smallest and on the largest scales that compose our universe. And, you know, we're sort of sitting somewhere perhaps in the middle of it, and it's really remarkable that creatures like us, who, you know, evolved from you know, small cells billions of years ago. Have come to the point where our ability to understand both the largest and the smallest scales together. Okay?